The present invention relates to nucleic acid molecules encoding an alternansucrase. Moreover, this invention relates to vectors, host cells and plant cells transformed with the herein-described nucleic acid molecules, and plants containing said cells. Moreover, methods for preparing transgenic plants which due to the insertion of DNA molecules encoding an alternansucrase, synthesize the carbohydrate alternan, are described. Furthermore, methods for preparing alternan are described.
Prior art documents, the disclosure content of which is included into the present application by reference thereto, are cited hereinafter.
Alternan is a polysaccharide composed of glucose units. The glucose units are linked to each other via xcex1-1,3- and xcex1-1,6-glycosidic bonds, and said two types of bonds predominantly appear alternatingly. However, alternan is not a linear polysaccharide, but may contain branches (Seymour et al., Carbohydrate Research 74, (1979), 41-62). Because of its physico-chemical properties, the possibilities of application of alternan both in the pharmaceutical industry, for instance as a carrier of pharmaceutically active ingredients and as an additive in the textile, cosmetics and food industry have been discussed (Lopez-Munguia et al., Enzyme Microb. Technol. 15, (1993), 77-85; Leathers et al., Journal of Industrial Microbiology and Biotechnology 18, (1997), 278-283). Moreover, it can be used as a substitute for gum arabic (Cotxc3xa9, Carbohydrate Polymers 19, (1992), 249-252).
Industry has a high interest in biotechnological methods for preparing oligosaccharides and polysaccharides, and in particular alternan which is hardly or not at all accessible to classical organic synthesis. Compared to the classical approach of organic synthesis chemistry, biotechnological processes offer advantages. For instance, enzymatically catalyzed reactions as a rule show much higher specificities (regio specificity, stereo specificity) and higher reaction speeds, proceed under milder reaction conditions and lead to higher yields. These factors are of outstanding importance in the preparation of new oligosaccharides and polysaccharides.
Alternan is prepared enzymatically with the use of enzymes possessing the biological activity of alternansucrases. Alternansucrases belong to the group of glucosyltransferases, which, starting from saccharose, are able to catalyze the formation of alternan and fructose. So far, alternansucrases have only been found in the bacterium Streptococcus mutans (Mukasa et al. (J. Gen. Microbiol. 135 (1989), 2055-2063); Tsumori et al. (J. Gen. Microbiol. 131 (1985), 3347-3353)) and in specific strains of the gram positive bacterium Leuconostoc mesenteroides where they are, as a rule, present together with other polysaccharide-forming enzymes, such as for instance dextran-forming dextransucrases, or together with polysaccharide-degrading enzymes, such as alternanases. Hence, the naturally occurring strains also produce dextran in addition to alternan.
So far, alternan has been prepared in a cell-free system using partially purified proteins or by fermentation using alternansucrase-producing strains of Leuconostoc mesenteroides. 
Various purification methods for the purification of alternansucrases have been described (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85; Lopez-Munguia et al., Annals New York Academy of Sciences 613 (1990), 717-722; Cotxc3xa9 and Robyt, Carbohydrate Research 101 (1982), 57-74). These methods are complex and relatively costly, and, as a rule, lead to low protein yields (Leathers et al., Journal of Industrial Microbiology and Biotechnology 18 (1997), 278-283). None of these methods allows highly pure alternansucrase protein to be produced, and therefore sequencing of the protein and the isolation of the corresponding DNA sequences have not been successful so far. If the alternansucrase protein purified according to these methods is used for in vitro preparation of alternan, then the dextransucrase protein residues contained in the alternansucrase preparation produce dextran impurities in the alternan produced. The separation of alternan and dextran is relatively time-consuming and costly (Leathers et al., Journal of Industrial Microbiology and Biotechnology 18 (1997), 278-283). Another disadvantage of the dextransucrase protein impurities contained in the enzyme preparation of alternansucrase protein is the fact that a part of the saccharose substrate is converted into dextran and not into alternan, which results in a reduction of the alternan yield.
The fermentative preparation by means of Leuconostoc also leads to the formation of product mixtures of alternan and dextran. In order to increase the amount of alternansucrase from Leuconostoc strains, mutants have been isolated, such as the mutant NRRL B-21138, which secrete the alternansucrase and lead to a higher proportion of the amount of alternansucrase formed relative to dextransucrase. However, if such mutants are fermented with sucrose, the alternan obtained continues to show dextran impurities (Leathers et al., Journal of Industrial Microbiology and Biotechnology 18 (1997), 278-283).
As can be seen from the prior art discussed above, it has not been possible to provide highly purified alternansucrase protein so far.
Hence, the present invention addresses the problem of providing means and methods allowing alternan to be prepared in a time-saving and inexpensive manner.
This problem is solved by the provision of the embodiments characterized in the patent claims.
Consequently, the present invention relates to a nucleic acid molecule encoding a protein possessing the biological activity of an alternansucrase selected from the group consisting of
(a) nucleic acid molecules encoding at least the mature form of a protein which comprises the amino acid sequence indicated in Seq. ID No. 2 or the amino acid sequence encoded by the cDNA contained in plasmid DSM 12666;
(b) nucleic acid molecules comprising the nucleotide sequence indicated in Seq. ID No. 1 or the nucleotide sequence of the cDNA contained in plasmid DSM 12666 or a corresponding ribonucleotide sequence;
(c) nucleic acid molecules encoding a protein, the amino acid sequence of which has a homology of at least 40% to the amino acid sequence indicated in Seq. ID No. 2;
(d) nucleic acid molecules, one strand of which hybridizes with the nucleic acid molecules as defined in (a) or (b);
(e) nucleic acid molecules comprising a nucleotide sequence encoding a biologically active fragment of the protein which is encoded by any one of the nucleic acid molecules as defined in (a), (b), (c) or (d); and
(f) nucleic acid molecules, the nucleotide sequence of which deviates because of the degeneration of the genetic code from the sequence of the nucleic acid molecules as defined in (a), (b), (c), (d) or (e).
Consequently, the present invention relates to nucleic acid molecules encoding proteins possessing the biological activity of an alternansucrase, said molecules preferably encoding proteins comprising the amino acid sequence indicated in Seq. ID No.2.
An enzyme possessing the enzymatic or biological activity of an alternansucrase (E.C. 2.4.1.140) is understood to mean an enzyme which is able to catalyze the conversion of saccharose into alternan and fructose. This conversion may occur both in the presence and absence of external acceptors (for instance maltose, isomaltose, isomaltotriose etc.). In the absence of external acceptors, alternansucrases starting from saccharose catalyze the release of fructose and high molecular alternan, a polysaccharide composed of glucose units, the backbone of which consists of glucose units predominantly connected to each other alternatingly by xcex1-1,3- and xcex1-1,6-glycosidic bonds. Concerning the percentage of xcex1-1,3- and xcex1-1,6-linked glucose units the literature displays different values. According to Mukasa et al. (J. Gen. Microbiol. 135 (1989), 2055-2063), alternan consists of 76 mol % xcex1-1,3-linked glucose and 24 mol % xcex1-1,6-linked glucose. Tsumori et al. (J. Gen. Microbiol. 131 (1985), 3347-3353) describe alternan as a polyglucan containing 49.1 mol % xcex1-1,6-linked glucose and 33.9 mol % xcex1-1,3-linked glucose with 13.6 mol % terminal glucose and 3.3 mol % xcex1-1,3,6-branched glucose. In the presence of external acceptors, such as maltose, isomaltose, isomaltotriose and methyl-xcex1-D-glucan, alternansucrase can catalyze the synthesis of xcex1-D-glucan chains, in which the glucose residues are predominantly alternatingly connected by xcex1-1,6- and xcex1-1,3-glycosidic bonds, and the synthesis of fructose at these polysaccharide acceptors. Depending on the acceptor used, the products formed have different structures. The enzymatic activity of an alternansucrase can for instance be detected as described by Lopez-Munguia (Annals New York Academy of Sciences 613 (1990), 717-722) or as described in the examples of the present application.
The invention in particular relates to nucleic acid molecules containing the nucleotide sequence indicated under Seq. ID No. 1 or a part thereof, and preferably to molecules, which comprise the coding region indicated in Seq. ID No. 1 or corresponding ribonucleotide sequences.
Moreover, the present invention relates to nucleic acid molecules which encode an alternansucrase and the one strand of which hybridizes with one of the above-described molecules.
The present invention also relates to nucleic acid molecules which encode a protein, which has a homology, that is to say an identity of at least 40%, preferably at least 60%, preferably at least 70%, especially preferably at least 80% and in particular at least 90% to the entire amino acid sequence indicated in Seq. ID No. 2, the protein possessing the biological activity of an alternansucrase.
The present invention also relates to nucleic acid molecules, which encode an alternansucrase and the sequence of which deviates on account of the degeneration of the genetic code from the nucleotide sequences of the above-described nucleic acid molecules.
The invention also relates to nucleic acid molecules possessing a sequence which is complementary to the whole or a part of the above-mentioned sequences.
The nucleic acid sequence indicated in Seq. ID No. 1 for instance encodes an extracellular alternansucrase. Secretion is ensured by a signal sequence which comprises the first approximately 39 N-terminal amino acid groups of the Seq. ID No. 2. In certain circumstances it may be desirable for only the mature protein to be expressed without naturally occurring signal sequences and/or together with other signal sequences. Hence, the above-described nucleic acid molecules encode at least the mature form of a protein possessing the biological activity of an alternansucrase.
Within the present invention the term xe2x80x9chybridizationxe2x80x9d means hybridization under conventional hybridization conditions, preferably under stringent conditions, as for instance described in Sambrook et al., Molecular Cloning, A Laboratory Manual, 2nd edition (1989) Cold Spring Harbour Laboratory Press, Cold Spring Harbour, N.Y. Within an especially preferred meaning the term xe2x80x9chybridizationxe2x80x9d means that hybridization occurs under the following conditions:
Hybridization buffer: 2xc3x97SSC; 10xc3x97Denhardt solution (Fikoll 400+PEG+BSA; ratio 1:1:1); 0.1% SDS; 5 mM EDTA; 50 mM Na2HPO4;250 xcexcg/ml of herring sperm DNA; 50 xcexcg/ml of tRNA; or
0.25 M of sodium phosphate buffer, pH 7.2;
1 mM EDTA
7% SDS
Hybridization temperature T=60xc2x0 C.
Washing buffer: 2xc3x97SSC; 0.1% SDS
Washing temperature T=60xc2x0 C.
Nucleic acid molecules which hybridize with the nucleic acid molecules of the invention can, in principle, encode alternansucrases from any organism expressing such proteins.
Nucleic acid molecules which hybridize with the molecules of the invention can for instance be isolated from genomic libraries of microorganisms. Alternatively, they can be prepared by genetic engineering or chemical synthesis.
Such nucleic acid molecules may be identified and isolated with the use of the molecules of the invention or parts of these molecules or reverse complements of these molecules, for instance by hybridization according to standard methods (see for instance Sambrook et al., 1989, Molecular Cloning. A Laboratory Manual, 2nd edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). Nucleic acid molecules possessing the same or substantially the same nucleotide sequence as indicated in Seq. ID No. 1 or parts thereof can, for instance, be used as hybridization probes. The fragments used as hybridization probes can also be synthetic fragments which are prepared by usual synthesis techniques, and the sequence of which substantially coincides with that of an inventive nucleic acid molecule.
The molecules hybridizing with the nucleic acid molecules of the invention also comprise fragments, derivatives and allelic variants of the above-described nucleic acid molecules encoding an alternansucrase of the invention. Herein, fragments are understood to mean parts of the nucleic acid molecules which are long enough to encode one of the described proteins, preferably showing the biological activity of an alternansucrase. In this connection, the term derivative means that the sequences of these molecules also differ from the sequences of the above-described nucleic acid molecules in one or more positions and show a high degree of homology to these sequences. In this context, homology means a sequence identity of at least 40%, in particular an identity of at least 60%, preferably more than 80% and particularly preferably more than 90%. Deviations from the above-described nucleic acid molecules may have been produced by deletion, substitution, insertion and/or recombination.
Preferably, the degree of homology is determined by comparing the respective sequence with the nucleotide sequence of the coding region of SEQ ID No.1. When the sequences which are compared do not have the same length, the degree of homology preferably refers to the percentage of nucleotide residues in the shorter sequence which are identical to nucleotide residues in the longer sequence. The degree of homology can be determined conventionally using known computer programs such as the ClustalW program (Thompson et al., Nucleic Acids Research 22 (1994), 4673-4680) distributed by Julie Thompson (Thompson@EMBL-Heidelberg.DE) and Toby Gibson (Gibson@EMBL-Heidelberg.DE) at the European Molecular Biology Laboratory, Meyerhofstrasse 1, D 69117 Heidelberg, Germany. ClustalW can also be downloaded from several websites including IGBMC (Institut de Gxc3xa9nxc3xa9tique et de Biologie Molxc3xa9culaire et Cellulaire, B.P.163, 67404 Illkirch Cedex, France; ftp://ftp-iabmc.u-strasbg.fr/pub/) and EBI (ftp://ftp.ebi.ac.uk/pub/software/) and all sites with mirrors to the EBI (European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SD, UK).
When using ClustalW program version 1.8 to determine whether a particular sequence is, for instance, 90% identical to a reference sequence according to the present invention, the settings are set in the following way for DNA sequence alignments:
KTUPLE=2, TOPDIAGS=4, PAIRGAP=5, DNAMATRIX:IUB, GAPOPEN=10, GAPEXT=5, MAXDIV=40, TRANSITIONS: unweighted.
For protein sequence alignments using ClustalW program version 1.8 the settings are the following: KTUPLE=1, TOPDIAG=5, WINDOW=5, PAIRGAP=3, GAPOPEN=10, GAPEXTEND=0.05, GAPDIST=8, MAXDIV=40, MATRIX=GONNET, ENDGAPS(OFF), NOPGAP, NOHGAP.
Furthermore, homology means preferably that the encoded protein displays a sequence identity of at least 40%, more preferably of at least 60%, even more preferably of at least 80%, in particular of at least 90% and particularly preferred of at least 95% to the amino acid sequence depicted under SEQ ID NO: 2.
Homology, moreover, means that there is a functional and/or structural equivalence between the corresponding nucleic acid molecules or proteins encoded thereby. Nucleic acid molecules which are homologous to the above-described molecules and represent derivatives of these molecules are, as a rule, variations of these molecules which represent modifications having the same biological function. They may be either naturally occurring variations, for instance sequences from other microorganisms, or mutations, and said mutations may have formed naturally or may have been produced by deliberate mutagenesis. Furthermore, the variations may be synthetically produced sequences. The allelic variants may be naturally occurring variants or synthetically produced variants or variants produced by recombinant DNA techniques.
In a further preferred embodiment the term xe2x80x9cderivativexe2x80x9d encompasses a nucleic acid molecule coding for a protein which comprises at least one, more preferably at least three, even more preferably at least five, in particular at least ten and particularly preferred at least twenty of the peptide motifs selected from the group consisting of
a) MKOQE (SEQ ID NO: 22),
b) KKVPV (SEQ ID NO: 23),
c) KDDEN (SEQ ID NO: 24),
d) IDGNL (SEQ ID NO: 25),
e) YVADS (SEQ ID NO: 26),
f) HLRKN (SEQ ID NO: 27),
g) NENTP (SEQ ID NO: 28),
h) NVDGY (SEQ ID NO: 29),
i) NPDLK (SEQ ID NO: 30),
j) SNDSG (SEQ ID NO: 31),
k) NTFVK (SEQ ID NO: 32),
l) ISGYL (SEQ ID NO: 33),
m) SNAAL (SEQ ID NO: 34),
n) RQYTD (SEQ ID NO: 35),
o) QLYRA (SEQ ID NO: 36),
p) DDKAP (SEQ ID NO: 37),
q) TRQYT (SEQ ID NO: 38),
r) ITFAG (SEQ ID NO: 39),
s) NQYKG (SEQ ID NO: 40),
t) LFLNA (SEQ ID NO: 41),
u) QVSDT (SEQ ID NO: 42),
v) LITLN (SEQ ID NO: 43),
w) GRYVH (SEQ ID NO: 44),
x) TAPYG (SEQ ID NO: 45),
y) VVDYQ (SEQ ID NO: 46),
z) LSGQE (SEQ ID NO: 47).
The proteins encoded by the different variants of the nucleic acid molecules of the invention possess certain characteristics they have in common. These include for instance enzymatic activity, molecular weight, immunological reactivity, conformation, etc., and physical properties, such as for instance the migration behavior in gel electrophoreses, chromatographic behavior, sedimentation coefficients, solubility, spectroscopic properties, stability, pH optimum, temperature optimum etc.
Alternansucrase (E.C. 2.4.1.140) is an enzyme belonging to the group of glucosyltransferases. So far, alternansucrase activity has not been found in plants, but only in the bacterium Streptococcus mutans (Mukasa et al. (J. Gen. Microbiol. 135 (1989), 2055-2063); Tsumori et al. (J. Gen. Microbiol. 131 (1985), 3347-3353)) and in specific strains of the bacterium Leuconostoc mesenteroides, for instance in NRRL B-1355, NRRL B-1498 and NRRL B-1501. As a rule, these strains contain different glucosyltransferases and secrete dextransucrases apart from alternansucrases if they are allowed to grow on saccharose-containing media. As a rule, these two sucrases possess a high binding affinity to the polysaccharides synthesized by them (Lopez-Munguia et al., Annals New York Academy of Sciences 613 (1990), 717-722) with the result that these polysaccharides must be separated from the protein in the purification of the enzymes from Leuconostoc mesenteroides strains grown on saccharose-containing medium (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85; Leathers et al., Journal of Industrial Microbiology and Biotechnology 18 (1997), 278-283).
In the absence of external acceptors, alternansucrases, starting from saccharose, catalyze the release of fructose and high molecular alternan, a polysaccharide which is composed of glucose units, and the backbone of which consists of glucose units predominantly linked to each other alternatingly by xcex1-1,3- and xcex1-1,6-glycosidic bonds and which according to light scattering measurement data should have a molecular weight of  greater than 107 (Cotxc3xa9, Carbohydrate Polymer 19 (1992), 249-252). To date there has been no report of alternan possessing a terminal fructose residue. Nevertheless, the existence of a terminal fructose unit in alternan can not be completely excluded. Lopez-Munguia et al. (Enzyme Microb. Technol. 15 (1993) 77-85) describe that alternan is resistant to degradation by dextranases. However, it can be degraded by so-called alternanases, whereby ring-shaped oligomers of alternan of different polymerization degree can be produced (Biely et al., Eur. J. Biochem. 226 (1994), 633-639). Ultrasonic treatment of high molecular alternan allows the molecular weight of alternan to be reduced to  less than 106 (Cotxc3xa9, Carbohydrate Polymers 19 (1992), 249-252). If aqueous solutions of this ultrasonically treated alternan are prepared, then these solutions show rheological properties comparable to those of aqueous solutions of gum arabic. So-called xe2x80x9climit alternanxe2x80x9d having a molecular weight of about 3500 can be produced by enzymatic degradation using isomaltodextranase from Arthrobacter globiformis (NRRL B-4425) (Cote, Carbohydrate Polymers 19 (1992), 249-252).
In the presence of external acceptors, such as for instance maltose, isomaltose, isomaltotriose and methyl-xcex1-D-glucan, alternansucrase catalyzes at said saccharide acceptors the synthesis of xcex1-D-glucan chains, in which the glucose moieties are predominantly alternatingly linked by xcex1-1,6- and xcex1-1,3 glycosidic bonds, and the synthesis of fructose. Depending on the acceptor used, the resulting products have different structures and a molecular weight which is lower than that of high molecular alternan and a polymerization degree of  less than 15. Because of the polymerization degree, these products are often also referred to as oligoalternans (Pelenc et al., Sciences Des Aliments 11 (1991), 465-476). However, within the framework of the present invention these low molecular products which can be prepared in the presence of external acceptors are also to be referred to as alternan.
In the preparation of oligoalternans by means of partially purified alternansucrase protein, maltose is an acceptor (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85) producing high oligoalternan yields. Panose (degree of polymerization (d.p.) of 3) is the first acceptor product which is formed starting from maltose through the formation of an xcex1-1,6-glycosidic bond.
In contrast thereto, isomaltose is a less effective acceptor which leads to lower yields of oligoalternan (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85).
Alternansucrase is relatively stable and has a half life period of 2 days in 50 mM of acetate buffer, pH 5.4 at 40xc2x0 C. (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85). The enzyme shows maximum activity at a temperature of 40xc2x0 C. and a pH value of 5.6 (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85).
In the absence of the substrate saccharose, alternansucrase catalyzes disproportionation reactions leading to a (partial) rearrangement of alternan. In particular when partially purified alternansucrase preparations containing dextransucrase contaminations were used to prepare oligoalternans, high disproportionation rates were found which lead to a complete rearrangement of oligoalternan (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85). For the molecular weight of alternansucrase according to SDS PAGE determination, different numerical values can be found: 135 kDa, 145 kDa, 173 kDa and 196 kDa, respectively (Leathers et al., Journal of Industrial Microbiology and Biotechnology 18 (1997), 278-283; Kim and Robyt, Enzyme Microb. Technol. 16 (1994), 659-664; Zhanley and Smith, Applied and Environmental Microbiology 61(3) (1995), 1120-1123).
The enzymatic activity of an alternansucrase can be shown for instance as described in Lopez-Munguia et al. (Annals New York Academy of Sciences 613 (1990), 717-722) or as described in the examples of the present application. One activity unit (1u) can be defined as the amount of enzyme leading to the release of 1 xcexcmol of fructose within one minute.
The nucleic acid molecules of the invention can be DNA molecules, in particular genomic molecules. Moreover, the nucleic acid molecules of the invention may be RNA molecules. The nucleic acid molecules of the invention can be obtained for instance from natural sources or may be produced synthetically or by recombinant techniques.
The nucleic acid molecules of the invention allow host cells to be prepared which produce recombinant alternansucrase protein of high purity and/or in sufficient quantities, and genetically engineered plants possessing an activity of these enzymes leading to the formation of alternan in planta. Within the framework of the present invention the term xe2x80x9chigh purityxe2x80x9d means that the protein according to the invention displays a degree of purity of at least 80%, preferably of at least 90%, even more preferably of at least 95%. Moreover, means and methods are provided which may be used for preparing alternan using host cells and/or for preparing recombinant alternansucrase protein. Consequently, the provision of the nucleic acid molecules of the invention permits the preparation of alternan of high purity by methods which are relatively inexpensive and consume relatively little time.
In a preferred embodiment, the nucleic molecules of the invention are derived from microorganisms, preferably from bacteria, more preferably from gram-positive bacteria and in particular preferably from bacteria belonging to the genus Leuconostoc. Nucleic acid molecules from bacteria belonging to the species Leuconostoc mesenteroides are particularly preferred.
The invention also relates to oligonucleotides specifically hybridizing to a nucleic acid molecule of the invention. Such oligonucleotides have a length of preferably at least 10, in particular at least 15, and particularly preferably of at least 50 nucleotides. They are characterized in that they specifically hybridize to the nucleic acid molecules of the invention, that is to say that they do not or only to a very minor extent hybridize to nucleic acid sequences encoding other proteins, in particular other glucosyltransferases. The oligonucleotides of the invention can be used for instance as primers for amplification techniques such as the PCR reaction or as a hybridization probe to isolate related genes.
Moreover, the invention relates to vectors, in particular plasmids, cosmids, viruses, bacteriophages and other vectors commonly used in gene technology, which contain the above-described nucleic acid molecules of the invention. In a preferred embodiment of the invention, the vectors of the invention lend themselves to the transformation of fungal cells or cells of microorganisms. Preferably, such vectors are suitable to transform plant cells. Particularly preferably, such vectors permit the integration of the nucleic acid molecules of the invention, possibly together with flanking regulatory regions, into the genome of the plant cell. Examples thereof are binary vectors which can be used in the Agrobacteria-mediated gene transfer, and some are already commercially available.
In another preferred embodiment, the nucleic acid molecules contained in the vectors are connected to regulatory elements ensuring the transcription and synthesis of a translatable RNA in prokaryotic or eukaryotic cells.
The expression of the nucleic acid molecules of the invention in prokaryotic or eukaryotic cells, for instance in Escherichia coli, is interesting because it permits a more precise characterization of the enzymatic activities of the enzymes encoded by these molecules. Moreover, it is possible to express these enzymes in such prokaryotic or eukaryotic cells which are free from interfering enzymes, such as dextransucrases or other polysaccharide-forming or polysaccharide-degrading enzymes. In addition, it is possible to insert different mutations into the nucleic acid molecules by methods usual in molecular biology (see for instance Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2nd edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.), leading to the synthesis of proteins possibly having modified biological properties. On the one hand it is possible in this connection to produce deletion mutants in which nucleic acid molecules are produced by progressive deletions from the 5xe2x80x2 or 3xe2x80x2 end of the coding DNA sequence, and said nucleic acid molecules lead to the synthesis of correspondingly shortened proteins. Such deletions at the 5xe2x80x2 end of the nucleotide sequence for instance allow amino acid sequences to be identified which are responsible for the secretion of the enzyme in microorganisms (transit peptides). This permits the deliberate preparation of enzymes which are no longer secreted by the removal of the corresponding sequences, but remain within the cell of the corresponding host organism or are localized in other compartments, for instance in the plastids, mitochondria, vacuole, on account of the addition of other signal sequences.
On the other hand, the introduction of point mutations is also conceivable at positions at which a modification of the amino acid sequence for instance influences the enzyme activity or the control of the enzyme. In this manner, it is for instance possible to produce mutants which possess a modified stereo and regio selectivity or a modified Km value or which are no longer subject to the control mechanisms normally existing in the cell and realized via an allosteric control or covalent modification.
Moreover, mutants possessing a modified substrate or product specificity can be prepared. Furthermore, it is possible to prepare mutants having a modified activity-temperature-profile.
Furthermore, in the case of expression in plants, the insertion of mutations into the nucleic acid molecules of the invention allows the gene expression rate and/or the activity of the proteins encoded by the nucleic acid molecules of the invention to be increased.
For genetic engineering in prokaryotic cells, the nucleic acid molecules of the invention or parts of these molecules can be introduced into plasmids which permit mutagenesis or sequence modification by recombination of DNA sequences. Standard methods (see Sambrook et al., 1989, Molecular Cloning: A laboratory manual, 2nd edition, Cold Spring Harbor Laboratory Press, NY, USA) allow base exchanges to be performed or natural or synthetic sequences to be added. DNA fragments can be connected to each other by applying adapters and linkers to the fragments. Moreover, engineering measures which provide suitable restriction sites or remove surplus DNA or restriction sites can be used. In those cases, in which insertions, deletions or substitutions are possible, in vitro mutagenesis, xe2x80x9cprimer repairxe2x80x9d, restriction or ligation can be used. In general, a sequence analysis, restriction analysis and other methods of biochemistry and molecular biology are carried out as analysis methods.
Moreover, the invention relates to plasmid pAlsu-pSK (see FIG. 2 and Example 2) which was deposited at Deutsche Sammiung fur Mikroorganismen und Zelikulturen (DSMZ), Braunschweig, under the accession No. DSM 12666 on Feb. 4, 1999, and to the nucleic acid molecules contained in the insert of plasmid DSM 12666 and encoding a protein possessing the enzymatic activity of an alternansucrase. Moreover, the present invention also relates to nucleic acid molecules which hybridize to the insertion of plasmid DSM 12666. Also, the present invention relates to nucleic acid molecules the nucleotide sequence of which deviates from that of the nucleic acid molecules of the plasmid DSM 12666 insert, because of the degeneration of the genetic code. Furthermore, the present invention relates to nucleic acid molecules which have a homology, that is to say a sequence identity of at least 40%, preferably of at least 60%, more preferably of at least 80%, even more preferably of at least 90%, and most preferably of at least 95% to the sequence of the insertion of plasmid DSM 12666.
Another embodiment of the invention relates to host cells, in particular prokaryotic or eukaryotic cells transformed with an above-described nucleic acid molecule of the invention or with a vector of the invention, and to cells descended from such transformed cells and containing a nucleic acid molecule or vector of the invention.
According to another preferred embodiment, the host cells are cells of microorganisms. In the context of the present invention, the term xe2x80x9cmicroorganismxe2x80x9d comprises bacteria and all protists (e.g. fungi, in particular yeasts, algae) as defined Schlegel""s xe2x80x9cAligemeine Mikrobiologiexe2x80x9d (Georg Thieme Verlag, 1985, 1-2). A preferred embodiment of the invention relates to cells of algae and host cells belonging to the genera Aspergillus, Bacillus, Saccharomyces or Pichia (Rodriguez, Journal of Biotechnology 33 (1994), 135-146, Romanos, Vaccine, Vol. 9 (1991), 901 et seq.). A particularly preferred embodiment of the invention relates to E. coli cells. Alternansucrase is especially preferably secreted by the host cell. The preparation of such host cells for the production of recombinant alternansucrase can be carried out by methods known to a man skilled in the art. In a preferred embodiment of the invention, the host cells of the invention show no interfering enzymatic activities, such as those of polysaccharide-forming and/or polysaccharide-degrading enzymes.
An overview of different expression systems is for instance contained in Methods in Enzymology 153 (1987), 385-516, in Bitter et al. (Methods in Enzymology 153 (1987), 516-544) and in Sawers et al. (Applied Microbiology and Biotechnology 46 (1996), 1-9), Billman-Jacobe (Current Opinion in Biotechnology 7 (1996), 500-4), Hockney (Trends in Biotechnology 12 (1994), 456-463), Griffiths et al, Methods in Molecular Biology 75 (1997), 427-440). An overview of yeast expression systems is for instance given by Hensing et al. (Antonie van Leuwenhoek 67 (1995), 261-279), Bussineau et al. (Developments in Biological Standardization 83 (1994), 13-19), Gellissen et al. (Antonie van Leuwenhoek 62 (1992), 79-93, Fleer (Current Opinion in Biotechnology 3 (1992), 486-496), Vedvick (Current Opinion in Biotechnology 2 (1991), 742-745) and Buckholz (Bio/Technology 9 (1991), 1067-1072).
Expression vectors have been widely described in the literature. As a rule, they contain not only a selection marker gene and a replication-origin ensuring replication in the host selected, but also a bacterial or viral promoter, and in most cases a termination signal for transcription. Between the promoter and the termination signal there is at least one restriction site or a polylinker which enables the insertion of a coding DNA sequence. The DNA sequence naturally controlling the transcription of the corresponding gene can be used as the promoter sequence, if it is active in the selected host organism. However, this sequence can also be exchanged for other promoter sequences. It is possible to use promoters producing a constitutive expression of the gene and inducible promoters which permit a deliberate control of the expression of the postconnected gene. Bacterial and viral promoter sequences possessing these properties are described in detail in the literature. Regulatory sequences for the expression in microorganisms (for instance E. coli, S. cerevisiae) are sufficiently described in the literature. Promoters permitting a particularly high expression of the postconnected gene are for instance the T7 promoter (Studier et al., Methods in Enzymology 185 (1990), 60-89), lacUV5, trp, trp-lacUV5 (DeBoer et al., in Rodriguez and Chamberlin (Eds), Promoters, Structure and Function; Praeger, N.Y., (1982), 462-481; DeBoer et al., Proc. Natl. Acad. Sci. USA (1983), 21-25), Ip1, rac (Boros et al., Gene 42 (1986), 97-100). As a rule, the protein amounts are highest from the middle up to about the end of the logarithmic phase of the growth cycle of the microorganisms. Therefore, inducible promoters are preferably used for the synthesis of proteins. These promoters often lead to higher protein yields than do constitutive promoters. The use of highly constitutive promoters leads to the continuous transcription and translation of a cloned gene and thus often has the result that energy is lost for other essential cells functions with the effect that cell growth is slowed down (Bernard R. Glick/Jack J. Pasternak, Molekulare Biotechnologie (1995). Spektrum Akademischer Verlag GmbH, Heidelberg, Berlin, Oxford, p. 342). Therefore, in order to obtain an optimum amount of protein, a two-stage process is often used. First, the host cells are cultured under optimum conditions up to a relatively high cell density. In the second step, transcription is then induced depending on the type of promoter used. In this connection, a tac promoter is particularly suitable which can be induced by lactose or IPTG (=isopropyl-xcex2-D-thiogalactopyranoside) (deBoer et al., Proc. Natl. Acad. Sci. USA 80 (1983), 21-25). Termination signals for transcription are also described in the literature.
The transformation of the host cell with DNA encoding an alternansucrase can, as a rule, be carried out by standard methods, as for instance described in Sambrook et al., (Molecular Cloning: A Laboratory Course Manual, 2nd edition (1989) Cold Spring Harbor Press, New York; Methods in Yeast Genetics, A Laboratory Course Manual, Cold Spring Harbor Laboratory Press, 1990). The host cell is cultured in nutrient media meeting the requirements of the particular host cell used, in particular in respect of the pH value, temperature, salt concentration, aeration, antibiotics, vitamins, trace elements etc.
Moreover, the invention relates to proteins and biologically active fragments thereof, which are encoded by the nucleic acid molecules of the invention and to methods for their preparation, wherein a host cell according to the invention is cultured under conditions permitting the synthesis of the protein, and the protein is subsequently isolated from the cultured cells and/or the culture medium.
According to a preferred embodiment of the invention, the alternansucrase is a recombinantly produced protein. In the context of the present invention, this is a protein prepared by inserting a DNA sequence encoding the protein into a host cell and expressing it therein. The protein can then be isolated from the host cell and/or the culture medium.
The nucleic acid molecules of the invention now allow host cells to be prepared which produce recombinant alternansucrase protein of high purity and/or in sufficient amounts. Within the framework of the present invention the term xe2x80x9chigh purityxe2x80x9d means that the protein according to the invention displays a degree of purity of at least 80%, preferably of at least 90%, even more preferably of at least 95%. The time-consuming and costly methods already mentioned above, whereby alternansucrase protein which to date can only be obtained from particular Leuconostoc strains can be purified from other components such as for instance dextransucrases, polysaccharides, are dispensed with, because alternansucrase can be produced in host cells not possessing any adverse polysaccharide-synthesizing activities. Moreover, host cells and vectors can also be used, which allow the alternansucrase protein to be produced in the absence of saccharose, with the result that an additional separation of the alternansucrase protein from polysaccharides is no longer necessary. Moreover, the selection of suitable host cells and vectors allows alternansucrase protein to be provided in sufficient amounts, which has not been possible with the systems so far described.
Alternansucrase produced by the host cells can be purified by conventional purification methods, such as precipitation, ion exchange chromatography, affinity-chromatography, gel filtration, HPLC Reverse Phase Chromatography etc. The modification of the nucleic acid molecules of the invention encoding an alternansucrase and expressed in the host cells, allows to produce a polypeptide in the host cell which is easier to isolate from the culture medium because of particular properties. Thus, the protein to be expressed can be expressed as a fusion protein with an additional polypeptide sequence, the specific binding properties of which permit the isolation of the fusion protein by affinity chromatography (e.g. Hopp et al., Bio/Technology 6 (1988), 1204-1210; Sassenfeld, Trends Biotechnol. 8 (1990), 88-93).
Another embodiment of the invention relates to proteins possessing the enzymatic activity of an alternansucrase, in particular that from microorganisms, preferably Gram-positive microorganisms, particularly microorganisms of the genus Leuconostoc, and particularly preferably that from Leuconostoc mesenteroides. The molecular weight of the protein indicated in Seq. ID No. 2, as determined by calculation, is 228.96 kDa. The invention also relates to alternansucrases which possess a molecular weight of 229 kDaxc2x1120 kDa, preferably 229 kDaxc2x150 kDa, and particularly preferably 230 kDaxc2x125 kDa. The molecular weight of the mature protein, as determined by calculation, is 224.77 kDa.
The provision of the nucleic acid molecules of the invention, for the first time, makes it possible to prepare alternansucrase-expressing plant cells by means of genetic engineering, which was not possible so far, because classical culturing methods do not allow bacterial and fungal genes to be expressed in plants.
The invention, therefore, also relates to transgenic plant cells transformed by a nucleic acid molecule of the invention or a vector of the invention or descended from such cells, the nucleic acid molecule which encodes the protein that has the biological activity of an alternansucrase being under the control of regulatory elements permitting the transcription of a translatable mRNA in plant cells.
The introduction of the activity of the proteins of the invention, for instance by expression of corresponding nucleic acid molecules, opens the possibility of producing alternan in plant cells correspondingly modified by genetic engineering. Hence, the expression of the nucleic acid molecules of the invention in plant cells is possible, allowing an additional, corresponding alternansucrase activity not present in the wild type to be introduced. Moreover, it is possible to modify the nucleic acid molecules of the invention according to methods known to a skilled person, in order to obtain alternansucrases of the invention which for instance possess modified temperature dependencies or substrate or product specificities. Such methods have already been described in more detail in a different context above.
A plurality of techniques is available by which DNA can be inserted into a plant host cell. These techniques include the transformation of plant cells by T-DNA using Agrobacterium tumefaciens or Agrobacterium rhizogenes as a transforming agent, the fusion of protoplasts, injection, electroporation of DNA, insertion of DNA by the biolistic approach and other possibilities.
The use of the Agrobacteria-mediated transformation of plant cells has been extensively investigated and sufficiently described in EP 120 516; Hoekema, In: The Binary Plant Vector System, Offsetdrukkerij Kanters B. V., Alblasserdam (1985), Chapter V; Fraley et al, Crit. Rev. Plant Sci. 4 (1993), 1-46 and An et al., EMBO J. 4 (1985), 277-287. Regarding the transformation of potatoes see for instance Rocha-Sosa et al. (EMBO J. 8 (1989), 29-33).
The transformation of monocotyledonous plants by means of Agrobacterium-based vectors has also been described (Chan et al., Plant Mol. Biol. 22 (1993), 491-506; Hiei et al., Plant J. 6 (1994) 271-282; Deng et al, Science in China 33 (1990), 28-34; Wilmink et al, Plant Cell Reports 11 (1992), 76-80; May et al., Bio/Technology 13 (1995), 486-492; Conner and Dormisse, Int. J. Plant Sci. 153 (1992), 550-555; Ritchie et al. Transgenic Res. 2 (1993), 252-265). An alternative system for transforming monocotyledonous plants is the transformation by the biolistic approach (Wan and Lemaux, Plant Physiol. 104 (1994), 37-48; Vasil et al., Bio/Technology 11 (1993), 1553-1558; Ritala et al., Plant Mol. Biol. 24 (1994) 317-325; Spencer et al., Theor. Appl. Genet. 79 (1990), 625-631), protoplast transformation, electroporation of partially permeabilized cells, insertion of DNA via glass fibers. The transformation of maize in particular has been repeatedly described in the literature (see for instance WO 95/06128, EP 0 513 849, EP 0 465 875, EP 29 24 35; Fromm et al, Biotechnology 8, (1990), 833-844; Gordon-Kamm et al., Plant Cell 2, (1990), 603-618; Koziel et al., Biotechnology 11 (1993), 194-200; Moroc et al., Theor. Appl. Genet. 80, (1990), 721-726).
The successful transformation of other types of cereals has also been described for instance of barley (Wan and Lemaux, supra; Ritala et al., supra, Krens et al., Nature 296 (1982), 72-74) and wheat (Nehra et al., Plant J. 5 (1994), 285-297). Generally, any promoter active in plant cells is suitable to express the nucleic acid molecules in plant cells. The promoter can be so chosen that the expression in the plants of the invention occurs constitutively or only in a particular tissue, at a particular time of plant development or at a time determined by external influences. The promoter may be homologous or heterologous to the plant.
Suitable promoters are for instance the promoter of 35S RNA of the Cauliflower Mosaic Virus (see for instance U.S. Pat. No. 5,352,605) and the ubiquitin-promoter (see for instance U.S. Pat. No. 5,614,399) which lend themselves to constitutive expression, the patatin gene promoter B33 (Rocha-Sosa et al., EMBO J. 8 (1989), 23-29) which lends itself to a tuber-specific expression in potatoes or a promoter ensuring expression in photosynthetically active tissues only, for instance the ST-LS1 promoter (Stockhaus et al., Proc. Natl. Acad. Sci. USA 84 (1987), 7943-7947; Stockhaus et al., EMBO, J. 8 (1989) 2445-2451), the Ca/b-promoter (see for instance U.S. Pat. No. 5,656,496, U.S. Pat. No. 5,639,952, Bansal et al., Proc. Natl. Acad. Sci. USA 89 (1992), 3654-3658) and the Rubisco SSU promoter (see for instance U.S. Pat. No. 5,034,322; U.S. Pat. No. 4,962,028) or the glutelin promoter from wheat which lends itself to endosperm-specific expression (HMW promoter) (Anderson, Theoretical and Applied Genetics 96, (1998), 568-576, Thomas, Plant Cell 2 (12), (1990), 1171-1180), the glutelin promoter from rice (Takaiwa, Plant Mol. Biol. 30(6) (1996), 1207-1221, Yoshihara, FEBS Lett. 383 (1996), 213-218, Yoshihara, Plant and Cell Physiology 37 (1996), 107-111), the shrunken promoter from maize (Maas, EMBO J. 8 (11) (1990), 3447-3452, Werr, Mol. Gen. Genet. 202(3) (1986), 471-475, Werr, Mol. Gen. Genet. 212(2), (1988), 342-350), the USP promoter, the phaseolin promoter (Sengupta-Gopalan, Proc. Natl. Acad. Sci. USA 82 (1985), 3320-3324, Bustos, Plant Cell 1 (9) (1989), 839-853) or promoters of zein genes from maize (Pedersen et al., Cell 29 (1982), 1015-1026; Quatroccio et al., Plant Mol. Biol. 15 (1990), 81-93). However, promoters which are only activated at a point in time determined by external influences can also be used (see for instance WO 93/07279). In this connection, promoters of heat shock proteins which permit simple induction may be of particular interest. Moreover, seed-specific promoters such as the USP promoter from Vicia faba which ensures a seed-specific expression in Vicia faba and other plants may be used (Fiedler et al., Plant Mol. Biol. 22 (1993), 669-679; Bxc3xa4umlein et al., Mol. Gen. Genet. 225 (1991), 459-467). Moreover, fruit-specific promoters, such as described in WO 91/01373 may be used too.
Moreover, a termination sequence may be present, which serves to terminate transcription correctly and to add a poly-A-tail to the transcript, which is believed to have a function in the stabilization of the transcripts. Such elements are described in the literature (see for instance Gielen et al., EMBO J. 8 (1989), 23-29) and can be replaced at will.
Such cells can be distinguished from naturally occurring plant cells inter alia by the fact that they contain a nucleic acid molecule of the invention which does not naturally occur in these cells. Moreover, such transgenic plant cells of the invention can be distinguished from naturally occurring plant cells in that they contain at least one copy of the nucleic acid molecule of the invention stably integrated in their genome.
Moreover, the plant cells of the invention can preferably be distinguished from naturally occurring plant cells by at least one of the following features: If the inserted nucleic acid molecule of the invention is heterologous to the plant cell, then the transgenic plant cells are found to have transcripts of the inserted nucleic acid molecules of the invention. The latter can be detected for instance by Northern blot analysis. The plants cells of the invention preferably contain a protein encoded by an inserted nucleic acid molecule of the invention. This can be shown for instance by immunological methods, in particular by Western blot analysis.
Transgenic plant cells can be regenerated to whole plants according to methods known to a person skilled in the art.
The present invention also relates to the plants obtainable by regeneration of the transgenic plant cells of the invention. Furthermore, it relates to plants containing the above-described transgenic plant cells.
In most plants, the photoassimilates in the form of sugars formed during photosynthesis within a plant, i.e. mainly in the form of saccharose, are transported to the corresponding target organs. As saccharose is the substrate of the polymerization reaction of alternansucrase, all plants, both monocotyledonous and dicotyledonous can, in principle, be modified by the nucleic acid molecule of the invention in respect of alternansucrase expression.
The expression in plants of the nucleic acid molecules of the invention encoding a protein having the enzymatic activity of an alternansucrase can, for instance, be used to achieve a modification of the viscosity of the extracts possibly obtained from the plants, said modification being achieved by the synthesis of alternan. In this connection, for instance tomatoes are of interest. The expression of an alternansucrase in a tomato fruit leads to the synthesis of alternan and results in a modification of the viscosity of extracts obtained from these fruits for instance for the production of tomato puree or tomato ketchup.
The expression of the nucleic acid molecules of the invention is in particular advantageous in those organs of the plant which show a higher saccharose content or store saccharose. Such organs are for instance the beet of sugar beet or the cane of sugar cane. As these plants normally do not store any appreciable amounts of starch, the alternans synthesized by the alternansucrase from these plants could be isolated in the pure form.
The site where the biosynthesis of the saccharose in the plant cell occurs is the cytosol. The storage site, however, is the vacuole. During its transport into the storage tissue of the sugar beet or the potato or during its transport into the endosperm of seeds, the saccharose must pass the apoplast. Hence, all three compartments, i.e. the cytosol, the vacuole, the apoplast, lend themselves to the expression of the nucleic acid molecules for the synthesis of alternan. In addition, the plastids also lend themselves thereto, as could for instance be shown by the expression of bacterial fructosyl transferases in amyloplasts. Said fructosyl transferases which likewise require saccharose as a substrate, were able to mediate the formation of xe2x80x9camylofructanxe2x80x9d in amyloplasts (Smeekens, Trends in Plant Science, Vol. 2, No. 8 (1997), 286-288).
In the case of starch-producing plants, such as potatoes and maize, where the starch biosynthesis and starch storage normally take place in the amyloplasts, an expression of the alternansucrase in apoplasts, in the cytosol or in the vacuole would lead to an additional synthesis of oligosaccharides and/or polysaccharides in these compartments, which can mean an overall increase in the yield.
As in the case of potatoes the starch synthesized in the amyloplasts can be separated from the alternan synthesized in the apoplast, in the cytosol or in the vacuole, the very same plant can be used to recover starch and alternan.
Moreover, transgenic potato and maize plants are known, the starch synthesis of which in the tubers and grains, respectively, is completely inhibited due to the inhibition of ADP-glucose-pyrophosphorylase by an antisense construct. In the case of potatoes, soluble sugars, in particular saccharose and glucose, accumulate instead, for instance in the tubers (Mxc3xcller-Rxc3x6ber et al., EMBO J. 11 (1992), 1229-1238). Alternan can be prepared in the cytosol, the vacuole or apoplast of these plants by the expression of an alternansucrase which uses saccharose as a substrate.
Therefore in another embodiment of the invention the plant cells of the invention are further characterized by a reduced ADP glucose pyrophosphorylase (AGPase) activity compared to corresponding cells from wild-type plants.
DNA molecules encoding AGPase are well known to the person skilled in the art and described for example in Mxc3xcller-Rxc3x6ber et al. (Mol. Gen. Genet. 224 (1) (1990), 136-146). By using DNA molecules encoding an AGPase it is possible to produce plants by means of recombinant DNA techniques (for example by an antisense, a ribozyme or a cosuppression approach) showing a reduced AGPase activity. Furthermore AGPase mutants, for example from maize (brittle-2 and shrunken-2), with reduced AGPase activity are known to the person skilled in the art.
The term xe2x80x9creducedxe2x80x9d means preferably a reduction of AGPase activity of at least 10%, more preferably of at least 50% and even more preferably of at least 80% in comparison to corresponding wild-type cells.
The activity of an AGPase can be determined according to Mxc3xcller-Rxc3x6ber et al. (Mol. Gen. Genet. 224 (1) (1990), 136-146) or to methods known to a person skilled in the art.
The reaction which is catalyzed by alternansucrase is distinguished by the fact that a glucose moiety is transferred directly from saccharose to an existing carbohydrate acceptor. By contrast, in the case of plants, the biosynthesis of linear glucans from saccharose, proceeds in such a way that the saccharose is first separated into glucose and fructose, which are then each converted into activated intermediate ADP-glucose. The glucose moiety is transferred by the enzyme starch-synthase from the ADP glucose to an already existing glucan, whereby ADP is released. The conversion of saccharose into two ADP glucose molecules requires several energy consuming reactions. Therefore, the energy consumption of the reaction catalyzed by alternansucrase is substantially lower than the energy consumption in the synthesis of polysaccharides from saccharose in plant cells, which can lead to an increased yield of synthesized oligo and/or polysaccharides in plants containing the nucleic acid molecules of the invention.
In the expression of the nucleic acid molecules in plants there exists in principle the possibility that the synthesized protein can be localized in any compartment of the plant cell (e.g. in the cytosol, plastids, vacuole, mitochondria) or the plant (e.g. in the apoplast). In order to achieve the localization in a particular compartment, the coding region must, where necessary, be linked to DNA sequences ensuring localization in the corresponding compartment. The signal sequences used must each be arranged in the same reading frame as the DNA sequence encoding the enzyme.
In order to ensure the location in the plastids it is conceivable to use one of the following transit peptides: of the plastidic Ferredoxin: NADP+ oxidoreductase (FNR) of spinach which is enclosed in Jansen et al. (Current Genetics 13 (1988), 517-522). In particular, the sequence ranging from the nucleotides xe2x88x92171 to 165 of the cDNA Sequence disclosed therein can be used, which comprises the 5xe2x80x2non-translated region as well as the sequence encoding the transit peptide. Another example is the transit peptide of the waxy protein of maize including the first 34 amino acid residues of the mature waxy protein (Klxc3x6sgen et al., Mol. Gen. Genet. 217 (1989), 155-161). It is also possible to use this transit peptide without the first 34 amino acids of the mature protein. Furthermore, the signal peptides of the ribulose bisposphate carboxylase small subunit (Wolter et al., Proc. NatI. Acad. Sci. USA 85 (1988), 846-850; Nawrath et al., Proc. Natl. Acad. Sci. USA 91 (1994), 12760-12764), of the NADP malate dehydrogenase (Gallardo et al., Planta 197 (1995), 324-332), of the glutathione reductase (Creissen et al., Plant J. 8 (1995), 167-175) or of the R1 protein Lorberth et al. (Nature Biotechnology 16, (1998), 473-477) can be used.
In order to ensure the location in the vacuole it is conceivable to use one of the following transit peptides: the N-terminal sequence (146 amino acids) of the patatin protein (Sonnewald et al., Plant J. 1 (1991), 95-106) or the signal sequences described by Matsuoka und Neuhaus, Journal of Experimental Botany 50 (1999), 165-174; Chrispeels und Raikhel, Cell 68 (1992), 613-616; Matsuoka und Nakamura, Proc. Natl. Acad. Sci. USA 88 (1991), 834-838; Bednarek und Raikhel, Plant Cell 3 (1991), 1195-1206; Nakamura und Matsuoka, Plant Phys. 101 (1993), 1-5.
In order to ensure the location in the mitochondria it is for example conceivable to use the transit peptide described by Braun et al.(EMBO J. 11, (1992), 3219-3227). In order to ensure the location in the apoplast it is conceivable to use one of the following transit peptides: signal sequence of the proteinase inhibitor 11-gene (Keil et al., Nucleic Acid Res. 14 (1986), 5641-5650; von Schaewen et al., EMBO J. 9 (1990), 30-33), of the levansucrase gene from Erwinia amylovora (Geier and Geider, Phys. Mol. Plant Pathol. 42 (1993), 387-404), of a fragment of the patatin gene B33 from Solanum tuberosum, which encodes the first 33 amino acids (Rosahl et al., Mol Gen. Genet. 203 (1986), 214-220) or of the one described by Oshima et al. (Nucleic Acid Res. 18 (1990), 181).
The nucleic acid sequence indicated in Seq. ID No. 1 encodes an extracellular alternansucrase. Secretion is ensured by a signal sequence comprising the first approximately 39 N-terminal amino acid residues of the Seq. ID No. 2.
The transgenic plants may, in principle, be plants of any plant species, that is to say they may be monocotyledonous and dicotyledonous plants. Preferably, the plants are useful plants cultivated by man for nutrition or for technical, in particular industrial, purposes. They are preferably starch-storing plants, for instance cereal species (rye, barley, oat, wheat, millet, sago etc.), rice, pea, marrow pea, cassava and potato; tomato, rape, soybean, hemp, flax, sunflower, cow pea or arrowroot, fiber-forming plants (e.g. flax, hemp, cotton), oil-storing plants (e.g. rape, sunflower, soybean) and protein-storing plants (e.g. legumes, cereals, soybeans). The invention also relates to fruit trees and palms. Moreover, the invention relates to forage plants (e.g. forage and pasture grasses, such as alfalfa, clover, ryegrass) and vegetable plants (e.g. tomato, lettuce, chicory) and ornamental plants (e.g. tulips, hyacinths). Sugar-storing and/or starch-storing plants are preferred. Sugar cane and sugar beet,. and potato plants, maize, rice, wheat and tomato plants are particularly preferred.
A further subject of the invention is a method for the production of transgenic plant cells and transgenic plants which in comparison to non-transformed wildtype cells/non-transformed wildtype plants synthesize alternan. In this method the expression and/or the activity of proteins encoded by the nucleic acid molecules of the invention is increased in comparison to corresponding wild-type cells/wildtype plants which do not show any alternansucrase expression and/or activity. In particular, such a method comprises the expression of a nucleic acid molecule according to the invention in plant cells. The nucleic acid molecule according to the invention is preferably linked to a promoter ensuring expression in plant cells. In a particularly preferred embodiment the method comprises the introduction of a nucleic acid molecule according to the invention into a plant cell and regeneration of a plant from this cell.
Such an increase in expression may, e.g., be detected by Northern blot analysis. The increase in activity may be detected by testing protein extracts for their alternansucrase activity derived from plant cells. The enzymatic activity of an alternansucrase can be measured, for instance, as described in Lopez-Munguia et al. (Annals New York Academy of Sciences 613, (1990), 717-722) or as described in the examples of the present application.
The invention also relates to propagation material of the plants of the invention. The term xe2x80x9cpropagation materialxe2x80x9d comprises those components of the plant which are suitable to produce offspring vegetatively or generatively. Suitable means for vegetative propagation are for instance cuttings, callus cultures, rhizomes or tubers. Other propagation material includes for instance fruits, seeds, seedlings, protoplasts, cell cultures etc. The preferred propagation materials are tubers and seeds. The invention also relates to harvestable parts of the plants of the invention such as, for instance, fruits, seeds, tubers or rootstocks.
Another embodiment of the invention relates to methods for preparing alternan which comprise the step of extracting and isolating alternan from a plant of the invention.
The extraction and isolation of alternan from a plant of the invention may be carried out by standard methods, such as precipitation, extraction and chromatographic methods.
Moreover, the present invention relates to alternan obtainable from a plant of the invention or from propagation material of the invention.
Moreover, the present invention relates to a method for preparing alternan and/or fructose, wherein a host cell of the invention secretes an alternansucrase into a saccharose-containing culture medium and alternan and/or fructose is/are isolated from the culture medium.
A preferred embodiment of the method of the invention uses an alternansucrase recombinantly produced and secreted by the host cell into the culture medium, thus avoiding the necessity of breaking up the cells and purifying the protein further, because the secreted protein can be obtained from the supernatant. The residual components of the culture medium can be removed by methods usual in processing technology, such as dialysis, reverse osmosis, chromatographic methods, etc. The same applies to the concentration of the protein secreted into the culture medium. The secretion of proteins by microorganisms is normally mediated by N-terminal signal peptides (signal sequence, leader-peptide, transit peptide). Proteins possessing this signal sequence are able to penetrate the cell membrane of the microorganism. A secretion of proteins can be achieved by adding the DNA sequence encoding this signal peptide to the corresponding region encoding the alternansucrase.
The natural signal peptide of the expressed alternansucrase is preferred, that of the alternansucrase from Leuconostoc mesenteroides NRRL B 1355 (see the first approximately 25 to 45 N-terminal amino acid residues of Seq. ID No. 2) is particularly preferred.
The signal peptide of xcex1-CGTase from Klebsiella oxytoca M5A1 (Fiedler et al., J. Mol. Biol. 256 (1996), 279-291) or a signal peptide as encoded by the nucleotides 11529-11618 of the sequence available under the GenBank accession number X86014 is most preferred.
The preparation of alternan and/or fructose requires neither activated glucose derivatives nor co-factors, as are necessary in most synthesis reactions for polysaccharides occurring within the cells. Hence, alternansucrase-secreting microorganisms can be cultured in saccharose-containing medium, the secreted alternansucrase leading to a synthesis of alternan and fructose in the culture medium.
Contrary to host cells from Leuconostoc mesenteroides, which secrete alternansucrase by nature, the host cells used according to the invention have the advantage that they do not secrete proteins possessing adverse polysaccharide-synthesizing side reactions, such as dextransucrase, with the result that outside the host cell, apart from alternan, no other polysaccharides can be formed which, as a rule, can be separated from alternan only by costly and time-consuming procedures. Moreover, the host cells according to a preferred embodiment of the invention do not have any adverse polysaccharide-degrading side activities, which would otherwise lead to losses in the yield of the alternan produced.
The method of the invention yields fructose apart from alternan. Fructose can be used for the inexpensive isolation of so-called xe2x80x9chigh-fructose-containing syrupsxe2x80x9d (HFCS). Conventional methods for preparing fructose on the one hand provide for the enzymatic break down of saccharose by means of an invertase or for the break down of starch into glucose units, mostly brought about by acid hydrolysis, and for subsequent enzymatic conversion of the glucose into fructose by glucose isomerases. However, both methods lead to mixtures of glucose and fructose. The two components must subsequently be separated from each other by chromatographic methods.
The separation of the two reaction products of the method of the invention, or the separation of the reaction products from the substrate saccharose can be achieved for example with the use of membranes permitting the penetration of fructose, but not the penetration of saccharose and/or alternans. If continuous removal of fructose via such a membrane is provided for, a more or less complete conversion of saccharose occurs.
The isolation of alternan and fructose can be carried out by standard methods or can be carried out as for instance described in the working examples.
According to one embodiment of the method, the host cells originate from microorganisms, preferably from Escherichia coli. 
In another embodiment, the method of the invention works with fungal host cells, in particular cells of yeasts, such as Saccharomyces cerevisiae. Yeast cells producing alternan in saccharose-containing medium because of the enzymatic activity of an alternansucrase, cannot be readily used, as yeasts secrete an invertase which breaks down the extracellular saccharose. The yeasts take up the resulting hexoses via a hexose transporter. However, one yeast strain has been described (Riesmeier et al. EMBO J. 11 (1992), 4705-4713) which carries a defective suc2 gene, and therefore cannot secrete invertase. Moreover, these yeast cells do not contain a transportation system able to import saccharose into the cells. If such a strain is so modified by means of the nucleic acid molecules of the invention that it secretes an alternansucrase into the culture medium, then fructose and alternan will be synthesized in saccharose-containing medium. The resulting fructose can subsequently be taken up by the yeast cells.
In another preferred embodiment of this method the host cell of the invention is present in an immobilized form.
As a rule, host cells are immobilized by inclusion of the cells in a suitable material, such as alginate, polyacrylamide, gelatin, cellulose or chitosan. However, adsorption or covalent binding of the cells to a carrier material is also possible (Brodelius and Mosbach, Methods in Enzymology Vol. 135 (1987), 222-230) An advantage of the immobilization of cells is that it allows substantially higher cell densities to be achieved than does culturing in liquid culture. This results in a higher productivity. Moreover, the costs for agitation and aeration of the culture decrease as do the costs for measures to maintain sterility. Another important aspect is the possibility of a continuous alternan production with the result that unproductive phases regularly occurring in fermentation processes can be avoided or at least greatly reduced.
Another embodiment of the invention relates to a method for preparing alternan and/or fructose, wherein
a) a saccharose-containing solution is contacted with a protein of the invention under conditions permitting the conversion of saccharose into alternan and/or fructose; and
b) alternan and/or fructose is/are isolated from the solution.
In this embodiment, the invention thus relates to a method for preparing alternan and/or fructose in vitro by means of a cell-free enzyme preparation. In this case, microorganisms which for instance secrete alternansucrase are cultured up to the stationary phase in a saccharose-free medium permitting the formation of alternansucrase protein. After removing the cells from the culture medium by centrifugation, the secreted enzyme can be recovered from the supernatant. The enzyme can subsequently be added to saccharose-containing solutions in order to synthesize alternan and/or fructose. Compared to the above-described synthesis of alternan in a system not freed from cells, this method offers the advantage that the reaction conditions can be controlled better and the reaction products are substantially purer and easier to purify. The purification of the protein can be carried out as already described above.
A preferred embodiment of the method of the invention uses a purified alternansucrase. Purified alternansucrase is understood to mean an enzyme which is largely free from cell components of the cells in which the protein is synthesized and shows no contamination with proteins possessing polysaccharide-synthesizing activities (e.g. dextransucrases) or degrading activities, and/or no contamination with (polysaccharide) acceptors. The term xe2x80x9cpurified alternansucrasexe2x80x9d preferably means an alternansucrase possessing a degree of purity of at least 70%, preferably at least 85%, and particularly preferably at least 95%.
The use of a purified protein for preparing alternan and/or fructose offers various advantages. Compared to methods working with partially purified protein extracts, the reaction medium of the method of the invention does not contain any residues of the production strain (microorganism) which is used for the purification of the protein or for its preparation by genetic engineering.
Moreover, the use of the purified protein is advantageous for food and pharmaceutical industry applications. Thanks to the fact that the reaction medium is defined in its composition and freed from all unnecessary components, the product is likewise more precisely defined in respect of its components. In consequence of this, the procedure for obtaining food and pharmaceutical industry approval of these products produced by genetic engineering requires substantially less documentation, especially since these products should not show any traces of a transgenic microorganism.
Moreover, contrary to the so far described in vitro methods in cell free systems using partially purified alternansucrase preparations, the method of the invention using a purified alternansucrase has the advantage that it allows highly pure alternan to be prepared without the occurrence of dextransucrase and dextran contaminations, because of the high purity of the protein of the invention. Moreover, the method of the invention permits the production of alternan in high yields, without losses caused for instance by adverse side reactions of a dextransucrase, which would convert part of the substrate saccharose into undesired dextran, the separation of which from alternan would only be possible using time-consuming and expensive methods.
The method of the invention produces fructose in addition to alternan. The fructose can be used for the inexpensive recovery of so-called xe2x80x9chigh-fructose-containing syrupsxe2x80x9d (HFCS). The method of the invention yields products of high purity, because of the use of a purified alternansucrase. Hence, compared to conventional methods for preparing HFCS from maize starch, which comprise costly process steps for removing the buffer salts by ion exchange, (Crabb and Mitchinson, TIBTECH 15 (1997), 349-352) the method of the invention does not require an expensive purification of the fructose.
Another preferred embodiment of the method of the invention uses a recombinantly prepared alternansucrase.
According to another preferred embodiment, the enzyme possessing the enzymatic activity of an alternansucrase is immobilized on a carrier material. Immobilization of the alternansucrase offers the advantage that the enzyme being the catalyst of the synthesis reaction can be easily recovered from the reaction mixture and reused several times. As the purification of enzymes is normally costly and time consuming, immobilization and reutilization of the enzymes allow for a substantial cost saving. Another advantage is the degree of purity of the reaction products not containing any residual protein.
There are many carrier materials available for the immobilization of proteins, and coupling to the carrier material can be made via covalent or non-covalent bonds (for an overview see: Methods in Enzymology 135, 136, 137). Widely used carrier materials include for instance agarose, alginate, cellulose, polyacrylamide, silica or nylon.
According to another embodiment of the invention, the alternansucrase (immobilized on a carrier material) is present between two membranes, one of which allows fructose, but not saccharose and alternan to penetrate, the other one of which allows saccharose, but not alternan to penetrate. The supply with substrate occurs through the membrane which allows saccharose to penetrate it. The synthesized alternan remains in the space between the two membranes and the released fructose can be continuously removed from the reaction equilibrium via the membrane which only allows fructose to penetrate it. Such an arrangement permits an efficient separation of the reaction products, and thus the production of pure fructose.
Moreover, the separation of fructose by ion exchange chromatography has been described (xe2x80x9cStarch Hydrolysis Products, Worldwide Technology, Production, and Applicationxe2x80x9d, Edited by F. W. Schenck, R. E. Hebeda, (1992), VCH Publishers, Inc., New York).
Thus, the use of alternansucrases for preparing pure fructose on the one hand involves the advantage that the relatively inexpensive substrate saccharose can be used as the starting material, and on the other hand the fructose can be isolated easily from the reaction mixture without additional enzymatic conversions or chromatographic methods.
Moreover, the invention relates to methods for preparing alternan and/or fructose, wherein
a) a saccharose-containing solution is contacted with a protein of the invention and acceptor molecules under conditions permitting the conversion of saccharose to alternan and/or fructose; and
b) alternan and/or fructose is/are isolated from the solution.
Within the framework of the present invention an acceptor molecule is understood to mean a molecule at which an alternansucrase is able to catalyze a chain-extending reaction. The acceptor which can be added to the reaction mixture at the beginning of the reaction is preferably a carbohydrate or a carbohydrate derivative. The use of external acceptors leads to the production of low molecular products which are to be designated alternan in the context of the present invention. The carbohydrate acceptor is preferably an oligo or polysaccharide, in particular a branched polysaccharide, such as dextrin, glycogen or amylopectin, preferably a linear polysaccharide, and particularly preferably a saccharide selected from the group consisting of maltose, isomaltose, isomaltotriose and methyl-xcex1-D-glucan. If an extension of the alternan chain at these acceptors occurs, then products are formed which have a higher molecular weight than the educt. Where maltose, isomaltose, isomaltotriose and methyl-xcex1-D-glucan are used, one obtains products which have a lower molecular weight than the alternan that can be prepared in the absence of external carbohydrate acceptors.
The size of the molecular weight of the oligoalternans prepared depends on the saccharose/acceptor ratio used. For instance the degree of polymerization of the products increases as the saccharose/isomaltose ratio increases.
Moreover, the saccharose/acceptor ratio has an influence on the oligoalternan yield. For instance, the oligoalternan yield increases as the saccharose/isomaltose ratio decreases.
The hitherto described methods for producing oligoalternan with the use of alternansucrases which the authors claim have been purified (Pelenc et al., Sciences Des Aliments 11 (1991), 465-476) only yielded product mixtures of oligoalternan and oligodextran, in the presence of the carbohydrate acceptor maltose. In this case, the synthesis of oligodextran is presumably attributable to dextransucrase-contaminations of the alternansucrase preparation. Compared to this method, the method of the invention offers the advantage that the use of recombinantly produced alternansucrase protein not containing any dextransucrase contaminants permits the preparation of oligoalternan without the simultaneous formation of oligodextran. Thus, the method of the invention makes it possible to provide oligoalternan, without requiring additional costly purification steps for separating oligodextran.
According to another preferred embodiment, the enzyme possessing the enzymatic activity of an alternansucrase is immobilized on a carrier material.
According to another preferred embodiment of the method of the invention, a recombinantly produced alternansucrase is used.
Moreover, the present invention relates to end products containing alternan. In this context, end products are understood to mean cosmetic products, preferably food products, fodder and particularly preferably pharmaceutical products.
Finally, the present invention relates to a method for preparing the afore-mentioned products comprising one of the above-described alternan manufacturing methods of the invention and the formulation of the thus obtained alternan in a form which is suitable for one of the afore-mentioned applications of the corresponding product.
These and other embodiments are disclosed and obvious to a skilled person and embraced by the description and the examples of the present invention. Additional literature regarding one of the above-mentioned methods, means and applications, which can be used within the meaning of the present invention, can be obtained from the state of the art, for instance from public libraries for instance by the use of electronic means. This purpose can be served inter alia by public databases, such as the xe2x80x9cmedlinexe2x80x9d, which are accessible via internet. Other databases and addresses are known to a skilled person and can be obtained from the internet. An overview of sources and information regarding patents and patent application in biotechnology is contained in Berks, TIBTECH 12 (1994), 352-364.